Electron gun for a tubular beam



Aug. 21, 1956 D. REVERDIN ELECTRON GUN FOR A TUBULAR BEAM Filed Nov. 27, 1951 A 1/ VI Fig 1 4 Sheets-Sheet 1 jZn/zlvrare,

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ELECTRON GUN FOR A TUBULAR BEAM Filed NOV. 27, 1951. 4 Sheets-Sheet 2 nix/J Aug. 21, 1956 D. REVERDIN 2,760,101

ELECTRON GUN FOR A TUBULAR BEAM Filed Nov. 27', 1951 4 Sheets-Sheet 3 jn/l/ Nrart.

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iinited States Patent ELECTRON GUN FOR A TUBULAR BEAM Daniel Reverdin, Paris, France, assignor to Compagnie Generale de Telegraphic Sans Fil, a corporation of France r Application November 27, 1951, Serial No. 258,334 Claims priority, application France November 30,1950

Claims. (1315-35 From the pending application filed by R. Warnecke et al. on February 16, 1950,,under Serial No. 144,501, amplifying tubes for very short waves are known, which operate by interaction between an electromagnetic wave travelling in a delay line made of two conductors at dif ferent potentials, and an electronic beam travelling perpendicularly to an electric field and a magnetic field crossed within an interaction space formed between the two conductors of that delay line, a feature of these tubes being that the delay line is composed of an axial conductor and an electrode concentric with this conductor, so

'2 streams of different velocities,;-moving perpendicularly to the crossed electric and magnetic fields.

Figure 1, in longitudinal section, represents a tube provided with a gun according to the invention. This tube includes an axial conductor 10f radius r0, supposed to be at zero. potential andsurrounded bygan anode 2 which is so made as to present the characteristics of delay, at least in the limits of the interaction space 3, due to a construction schematically illustrated by the dotted line. That anode of radius r1, is given a potential Vi supplied by the source 4, so that in the interaction space is developed a field:

1- log 1 1 m I wherein V10 is a constant and r the variable radius of any point. 1 By causing a suitable current I (supplied by a source 5) toflow through the axial conductor, a cylindrical symmetry field is produced in :the interaction space the value that the interaction space is cylindrical; then the magnetic field produced in that space is also of cylindrical symmetry and could be created, for example, by letting an intense direct current flow through the axial conductor. In these tubes at any point of the interaction space, the electric field and the magnetic field have both a value proportional to the inverse of the distance r between that point and the axis of the system, so that the ratio of the two fields is constant for every point of the space. From the copending application cylindrical emitting cathodes are also known, which are supported by the axial conductor and heated by the current producing the magnetic field.

The present invention refers to electronic guns producing tubular beams and particularly adapted to use in that type of tube. Its first object consists in the arrangement of a system of electronic optics, able to be formed in various embodiments, namely if the space charge is or is not taken into account, but in every case ensuring the effect of injection of a very focalized beam into the interaction space, with a velocity substantially equal to the ratio of the electric and magnetic fields established therein, which as it is known, determines the linearity of the electronic trojectories in the said space. Its second object consists in the application of one of these forms of embodiment to a tube working according to the same principle as described above, but different from the, tube described in the copending application by the fact that the in teraction between the energies in the beam and in the delay line is replaced by the interaction between electronic streams of different velocities in a manner known per se in other types of travelling wave tubes.

The invention will be better understood by considering the enclosed drawings in which:

Figures 1 and 2 are longitudinal sections through two non-limiting examples of embodiment of guns designed without taking the space charge into account.

Figure 3, a similar view of a non-limiting example 0 embodiment of gun designed taking the space charge into account.

Figure 4, a diagram for the graphic determination of certain constructive characteristics of the gun of Figure 3.

Figure 5, a view similar to Figures 1, 2 and 3 of an example of embodiment of the gun from Figure 3 incorporated in a travelling wave tube of tubular structure without any delay line and with interaction between electronic of which is:

F10 being a constant and no being vacuum permeability.

. might have a different value in the general case), this part extending outthrough the insulating base 7. On the interaction space side, the conductor 2 goes out through "the bottom of the envelope 2 holding the collector 21 for the electronic beam 8. The input and output for the electromagnetic wave travelling inthe delay line are represented in 22 and 23.

According to the invention, the electrons emitted by the cathode 6 are subjected to the action ofan electric field E2 and a magnetic field B2 in such a way that they be focalized in a very concentrated tubular beam 8 which travels along a trajectory corresponding to the half are of a curve of cycloidal appearance; then goes into the interaction space 3 with a velocity nearly equal to the ratio 131/31 and travels farther in that space according to a rectilinear trajectory of radius Tmax.

Thenecessary values of E2 and B2 could be chosen with the help of the following considerations:

Starting from the equations of movement of an electron of mass m and charge e, moving in the electric field E and the magnetic field B2, and neglecting the effects rmg=r exp (3) In order that the trajectories in the interaction space where an electric field E1 and a magnetic field B1 exist, be rectilinear, the following relation mustbe fulfilled:

. Y. E1 j t fi i Therefore we must have the relation As, from a'prac'tical point of view, it is convenient to make B1=Bz (the two fields being produced by the same magnetizing current I), it is necessary to make 2E2=E1 which means that the electric held near the gun must be equal to half the electric field in the 'interaction space.

A first example of embodiment of that relation, represented in Figure 1, makes use of an auxiliary electrode 9, of cylindrical form of radius r2 disposed around the cathode 6' and to which a potential V2 is given. The relation between 1'2 and V2 may be determined by considering that:

and by using the Relations 1 and 6. In the general case, the result is:

Y log r log t V2 2 log n-log r (8) In the case of Figure 1, r0 has been made equal "to ro':(ro=ro') with different values for V1 and V2. But it is also possible to avoid theuse of an auxiliary electrode when, according to Figure 2, the axial conductor and the anode have a transverse section variable in steps, the cathode being disposed on the thinner part of the axial conductor, and the anode part infront of this thinner part of the cathode being wider with respect to the anode part corresponding to the interaction space. In this case, V2=V1, and the relation between the radius m and f! is given by the Relation 8 in which re and re will have different values.

Moreover, this form of axial conductor, thinner on the gun side, presents the advantage that the conductor part in the interaction space does not heat as much as the part near the emitting source. In this case, the current generator of the magnetic field is usually sufficient to ensure the direct heating of the emitting coat laid on the surface, while in the Figure 1, it might be necessary to dispose a special heating circuit under the emitting coat.

The guns, as above described, designed without taking the space charge into account, can be used in amplifying tubes of small power, for example 0.5 kw. For greater powers, the elfect of space charge becomes important, the focalization becomes defective and the beam diverges. In this case, according to the invention, the focalization is improved by disposing Wehnelt electrodes on both sides of the cathodic cylinder, having conical surfaces advantageously incorporated in the surface of the axial conductor.

This form of embodiment of the invention is shown in Figure 3 where the elements similar to those of the preceding figures are found again under the some reference numbers. Adjacent on both sides of the cathode 6, are two conical surfaces 10 and 11 functioning as Wehnelt electrodes and joined to the surface of the axial conductor 1. The angles between these surfaces and the system axis may be determined graphically; in order to fulfill it, the form of the beam limit trajectory 12 is calculated by known mathematical methods and is then drawn on the diagram of Figure 4. The trajectories of every electron inside the beam are obtained by moving the limit trajectory parallel in the axial direction. According to the researches of the applicant, eifected on models sunk in an electrolytic tank, the optimum focalization takes place when the generatrices of the Wehnelt electrode conical surfaces form an angle of approximately 67.5" with the direction determined in drawing by a straight line passing by the starting point of the limit trajectory on the cathode, and the point of this trajectory whose ordinate corresponds to /6 of the maximum height reached by the trajectory. It is what is indicated in Figure 4 where the segment AB corresponds to the cathode surface and the points C and D correspond to /6 of the maximum height the limit trajectorieslZ and 13. The straight lines AC and BD then determine the directions from which the angles of 67.5 determining the positions of the Wehnelt electrode surfaces 10 and 11 must be counted. These straight lines are parallel, since the extreme trajectories have an identical form as stated above, therefore the angle between the generatrices of the Wehnelts 10 and 11 in any axial plan is The difference of radius of the axial conductor in the interaction space m with respect to the cathode radius r0 calculated on the basis of junction of the equipotential lines near the gun and in the interaction space, at the height Imax corresponding to the beam;

The Wehnelt surface 11, joining the cathode to the axial conductor in the interaction space, be profiled according to a progressive form as indicated in Figure 3. The conical surface of the Wehnelt 10, on the other side of the cathode, will be joined to a cylindrical surface whose radius would, for example, be equal to rmax. In orderftha't the electric field's may be correctly joined to each'other, in the region included between the Wehnelts, it is necessary to prolong the cylindrical anode 2 by a conical surface 14 widened towards the gun as shown in the figure.

With respect to the guns of Figures 1 and 2 designed without taking the space charge into account, the gun of Figure 3 presents the advantage that the Schottky noise is strongly limited by the space charge, and that the beam. focaliza'tion is better and independent from the current density. The gun will preferably be used in ultra short wave amplifying tubes of great power and constant output, where the maximum power, only limited by the space charge, will be reached without disturbing the focaliza'tion.

In any case, it is advantageous to use an oxide cathode as emitting cathode. In this case, the noise of internal origin is still decreased. Besides, if the cathode is directly heated by the current generating the magnetic field, and if this field must be adjusted by slight variations of this current, the temperature variations, in the case of an oxide cathode, are not sufficiently important for disturbing the emission with undesirable variations.

In the case Where the space charge in the tube is great, the law of the electric field variable inversely to the radius, according to the Formula 1, will no more be accurate. The ratio Ei/Bi will be different for the electrons of the outer surface and those of the inner surface of the tubular beam, but, as the space charge acts also upon the velocity of the electrons so that the velocity at the outer surface of the tubular beam be greater than the velocity at the inner surface, the equality between the velocity and the said ratio will be respected everywhere, and the progression of the electrons will remain nearly rectilinear Within the whole section of the beam.

The gun of Figure 3 allows 'very well of fulfilling such conditions, and a further object of the invention is the application of such a gun to a tube of the general type defined in the preamble, but in which the interaction between the energies of the beam and of the delay line is replaced by the interaction between electronic streams of different velocities.

Indeed, it is known that the conditions of unequality of velocity as above defined, are very favorable to the amplification by electronic wave. This principle has already been applied for constituting tubes without any delay line, similar either to the travelling wave tubes without any magnetic field, or to the tubes with crossed fields of rectilinear or circular form. The present invention however aims at tubes without any delay line, similar to the tubular tubes with crossed fields (the magneti'c field of which being of cylindrical symmetry), and for this purpose use is made of a gun of the type Shown inFigure 3.

In order to make an amplifying tube of that type, it is sufficient to inject a small amplitude wave in an input circuit and to gather the amplified wave in the output circuit. The width of amplification band in an electronic beam being relatively large, it will be advantageous to use input and output circuits covering. a wide band, as, for example, helix circuits. This advantage does not exclude the possibility of using other circuits, as, for example, coaxial inlets and outlets, since the tube has a coaxial form.

Figure shown gives a form of possible embodiment. The electrons are omitted by a gun of the type shown in Figure 3, composed of a cylindrical cathode 6 and of two Wehnelts 10 and 11. The interaction space 3 is contained between the axial conductor 1 and the tubular anode 2, which, differently from the known tubes of that type, is a simple cylinder without any delaying property. Two small pieces of helix and 16 are coupled respectively with the inlet and outlet to and from the beam of the interaction space, and a collector 17 absorbs the electrons which have returned a part of their alternative energy to the output circluit. The cathode, the two Wehnelts and the axial conductor are all at the same negative potential, while the anode is given a zero potential. A source 5 sends a current through the axial conductor, which heats the cathode and produces the magnetic field.

A circulation of water is necessary in the tube in order to absorb the heat produced by the current generating the magnetic field. This circulation passes by the channel 18 into the anode. In the case of a high power tube, the helix of the output circuit 16 should also be cooled. The connexions 19 and 20 couple the helices 15 and 16 to the input and output ultra high frequency circuits.

The essential advantage of this amplifier is its simplicity of construction. The number of electrodes is reduced to a minimum; the small pieces of helix form are short enough not to cause difliculties of mounting as for the tubes with helical delay line; the tubular construction without delay line requires only compact parts which are easy to machine.

What I claim is:

1. An electron tube of cylindrical structure comprising an axially positioned rectilinear conductor, an electron emitting cathode in the form of a cylinder, supported by said conductor near one end thereof and having an electron emissive lateral surface to emit electrons perpendicularly to said conductor, terminal connections to said conductor for establishing a direct current magnetic field having circular lines of force around said conductor, an anode electrode coaxial with said conductor, terminal connections to said anode electrode and said conductor for applying a potential difference therebetween to establish an electric field having radial lines of force crossed with said magnetic lines of force, an electron collector situated near the end of said conductor away from said cathode, and a system of electron-optical electrodes of rotational structure around said conductor, disposed near the initial part of the path of said electrons emitted perpendicularly to said conductor, to change the direction of said electrons and concentrate said electrons in a tubular beam having said conductor as an axis, said electrons moving between said conductor and said electrode perpendicularly to said crossed electric and magnetic lines of force towards said collector.

2. A tube according to claim 1, in which the system of electron optics includes a cylindrical electrode surrounding the cathode, means being provided for applying a predetermined potential thereto.

3. A tube according to claim 1, contained in a cylindrical metallic container, whose diameter, in front of the part of the axial conductor supporting the cathode, is larger than the diameter in front of the part of the said conductor extending between the cathode and the collector.

4. A tube according to claim 1, in which the diameter of the part of the axial conductor supporting the cathode is reduced with respect to the diameter of the part of the said conductor extending between the cathode and the collector.

5. A tube according to claim 4, in which the cathode is of the oxide type.

6. A tube according to claim 1, in which the bases of the cathodic cylinder are joined on both sides to substantially conical surfaces.

7. A tube according to claim 6, in which the angle between the outlines of an axial section of the said two conical surfaces is about 8. A tube according to claim 1, in which the diameter of the part of the axial conductor supporting the cathode is reduced with respect to the parts of the said conductor extending on each side of the said cathode, the junction between the said reduced part and the parts whose dimeter is larger being efiected by nearly conical surfaces.

9. An electron tube as claimed in claim 1, wherein said coaxial electrode is a delay line.

10. An electron tube as claimed in claim 1, wherein said coaxial electrode is a smooth cylinder having propagation characteristics for an ultra high frequency wave with an axial velocity substantially equal to light velocity, an input circuit and an output circuit for said wave being provided respectively near both extremities of said cylinder, said circuits being coupled with said tubular beam.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Bell System Tech. Jour. for January 1949, article by A. V. Hellenberg, pp. 52-58. 

